1. Field of the Invention
The present invention relates to a voltage balancer device for a battery pack composed of plural rechargeable secondary battery units connected in series or for a set of some secondary battery units adjacently connected, and the voltage balancer device is capable of adjusting the voltage of a secondary battery unit by discharging the voltage of the secondary battery unit.
2. Description of the Related Art
For example, a battery pack composed of a plurality of rechargeable secondary battery units (hereinafter, referred to as secondary battery units) connected in series is mounted on various types of vehicles such as a hybrid electric vehicle (HEV). In particular, such a HEV has a possibility of varying a voltage of each secondary battery mounted thereon. The voltage variation of each secondary battery unit in the HEV is caused by residual electric capacity of each secondary battery unit. The variation of the residual electric capacity is caused by various conditions such as the temperature variation and individual difference of each secondary battery unit. In order to eliminate the voltage variation between the secondary battery units, there is a conventional manner of charging all of the secondary battery units forming a battery pack by slightly over-discharging the battery pack composed of nickel-hydrogen battery.
On the contrary, a lithium battery as a secondary battery has a characteristic of promoting its deterioration by performing over-charging. If the over-charging is performed for such a lithium battery, the reliability thereof is drastically decreased.
Japanese patent laid open publication number JP2005-56654 has disclosed a voltage balancer device capable of adjusting the voltage of each secondary battery unit by detecting the voltage of each secondary battery unit and then by discharging a higher voltage of the secondary battery unit through a discharge path placed per secondary battery unit, where the discharge path is connected to the secondary battery unit in parallel. However, when an electric contact resistance of a connector that connects each secondary battery unit with each discharge path in the conventional voltage balancer device described above, the voltage of the secondary battery unit is determined by the sum of the voltage drop of the discharge path and the voltage drop of the connector. Accordingly, even if the voltage of the discharge path is decreased to a specified voltage level, the voltage value of the secondary battery unit involves an error equal to the voltage drop of the connector to the specified voltage value. It is necessary to increase the resistance value of the discharge path in order to reduce the amount of the error. However, this solution introduces that the current flowing through the discharge path becomes small and it takes a long period of time for discharging the secondary battery unit.
In general, not only a battery pack composed of a plurality of lithium battery units described above, but also a secondary battery capable of adjusting the voltage of each secondary battery unit by discharging a voltage of each secondary battery unit to a specified voltage have a common problem in which controlling a voltage of each secondary battery unit with high accuracy is contrary to the adjusting period of time necessary for adjusting the voltage of the secondary battery unit.